[wyldfire]

This device's manufacturer claims an accuracy of "± 100ppm or ± 7% of reading".

But there's a review on Amazon that describes someone getting three units and finding as much as 600ppm variation among them.

Maybe they need some kind of calibration or tuning? Or their manufacturing process is way out of control.

[detaro]

They have a calibration feature, of course most of the reviews fail to mention if the person understood how it worked and used it. (I personally find the mechanism a bit odd, but I guess it makes some sense if you assume some things about the environment)

[stackmad]

It definitely shows that this is one of the cheapest meters on the market. But the values seem to be realistic, when having all windows open it shows 450-460 ppm which is at least close to the global 400ppm ( and I live next to a road in a bigger city ).